The present invention relates to fuel tanks and more particularly to polymer fuel tanks, and a method for reducing permeation of gaseous fluids.
Historically, fuel tanks have been manufactured from stamped metal shell halves. Metal fuel tanks are not only expensive, but add weight to a vehicle and are subject to corrosion. To reduce cost, weight, and avoid corrosion problems associated with metal fuel tanks, manufacturers have switched to using polymer fuel tanks in vehicles.
Polymer or plastic fuel tanks are corrosion resistant and weigh less than metal fuel tanks. One problem with polymer fuel tanks is that fuel vapor may permeate through the polymeric container walls. A polymer fuel tank formed from a thermoplastic resin such as polyethylene or polypropylene can have fuel permeability up to several grams per day, releasing hydrocarbon molecules typically comprising aromatic, aliphatic, oxygenate, alcohol, etc. or mixtures thereof. Polymer fuel tanks made of other materials may release these and other components of fuel.
Manufacturers have attempted a variety of techniques to reduce the permeation of polymer fuel tanks. One technique used to improve the barrier properties of fuel tanks is to electroplate the polymeric fuel tank with a layer of copper, nickel, and chrome. The problem with electroplating a polymer fuel tank is that an electroplatable polymer must be used or the polymer must be pre-treated for electroplating. The electroplating of three successive layers of different metals is also expensive, time consuming, and the electroplated layers may be subject to corrosion.
Yet another way of reducing fuel vapor emissions through polymer fuel tanks is to use a primary vapor barrier layer such as an ethylene vinyl alcohol copolymer (EVOH). EVOH provides an effective barrier layer to prevent fuel emissions, is somewhat flexible, and is relatively inexpensive. One problem with EVOH is that while it has some flexibility it is too brittle for many applications and may be subject to degradation when exposed directly to fuels containing alcohol or other fuel additives. Therefore, EVOH barrier layers are commonly placed between layers of polymeric material. However, EVOH, like most primary barrier layers, is not able to bond to itself to form a continuous layer when formed shell halves are joined to form the fuel tank. Typically, when the flanges are heat welded together, polyethylene from each shell half is welded together to form the fuel tank. Because the layers of EVOH do not bond to each other, these fuel tanks allow fuel vapors to permeate in areas where the EVOH or primary vapor barrier layer does not meet or bind to itself. Polymer fuel tanks formed from polyethylene with a central EVOH barrier layer may transmit up to 50 mgs of fuel vapor per day.
A polymer fuel tank may include additional areas of permeation. After the shell of the fuel tank is formed, various items such as vapor valves, fill check valves, fill nipples, recirculation nipples, or fuel filler necks may be attached to the fuel tank. Even if the fuel tank contains a primary barrier layer, this layer is breached when cutting holes to attach the valves, nipples, and necks. Because these attached items are generally formed from the same polymeric materials as the fuel tank to allow them to be welded to the fuel tank, they provide a permeation pathway for fuel vapors to exit the tank.
Even if a barrier layer is added to the attached item, for example, to the fuel filler neck, permeation may still occur. Permeation occurs because the primary barrier layer in the fuel tank is not able to bond to the barrier layer in the attached item to form a continuous barrier layer. The resulting diffusion pathway near attached items is relatively short, allowing easy permeation. The amount of permeation is inversely proportional to the length of the dispersion pathway.